“So I think that there is undoubtedly significant ongoing risk which is at least as great as the threat to London in the Seventies when the IRA were active on the mainland,” Hill is cited as saying in an interview with Telegraph.

Speaking in his first major interview since his appointment, Hill said he was increasingly concerned that the imminent return of hundreds of native Britons, who left the country to fight for the Islamic State (IS, formerly ISIS/ISIL), may stir radical ideologies.

“It’s an enormous concern that large numbers – we know this means at least hundreds of British citizens who have left this country in order to fight – are now returning or may be about to return,” Hill said.

“It is possible to point to distinctions in terms of the mindset, organization and strategy of different terrorist groups and therefore it would be wrong to draw a simple comparison between Irish republicanism and the ideology of so-called Islamic State,” said Hill.

The Irish Republican Army (IRA) - designated a terrorist group by Britain - was central to 30 years of sectarian conflict in Northern Ireland that pitted Catholic nationalists seeking union with Ireland against British security forces and mainly Protestant loyalists determined to stay in the United Kingdom.

An end to violence by IRA guerrillas was a central plank of a 1998 peace accord that ended three decades of conflict in Northern Ireland and bombing campaigns in mainland Britain.